Transmission Electron Microscopy (TEM) is a widely used technique for examining a very small area at a particular depth within a specimen. A common example, of particular interest in the context of the present invention, is the thin layer of insulation that underlies the gate of a field effect transistor (FET). Examination of this area (known as the active region of the transistor) needs to be performed after the full device has been fabricated since defects may have been introduced into the active area as a result of the fabrication process itself.
A specimen suitable for TEM needs to be between about 200 and 5,000 Angstroms thick. With this in mind, it is first ground down to a thickness that is just sufficient to retain its mechanical integrity. A small portion may then be removed and attached to a suitable holder which then allows it to be further thinned down or the full specimen may be attached to a suitable support. Thinning then proceeds in a limited area within which the area of interest for TEM lies. Material removal in such small areas is usually achieved by means of ion milling which allows the controlled removal of material down to thicknesses of a few hundred Angstroms.
The present invention uses the second of the above two methods. However, in the case of performing TEM on the active region of an FET it has been found that this method, as described above, often fails because the gate oxide that is to be examined separates from the main body of the specimen and falls away. The present invention is directed to providing a method of TEM specimen preparation that is free of this problem.
We have been unable to find any prior art that teaches the approach used by the present invention. Several references were, however, found to be of interest. For example, Hosono (U.S. Pat. No. 5,093,572 March 1992) describes a method of combining both focussed ion beams (FIB) and scanning electron microscopy (SEM) in the same apparatus. This makes it possible to observe the progress of an ion milling operation without having to interrupt it.
Ohnishi et al. (U.S. Pat. No. 5,270,552 Decmeber 1993) teach a process for partially milling out a tiny section of a relatively large specimen. A probe is then brought in contact with the specimen and bonded to it by means of a tungsten layer that is selectively deposited in the probe's immediate vicinity, using a combination of chemical vapor deposition (CVD) and local heating. Once the probe has bonded to the main specimen, an additional milling step frees the section, making it available for TEM.
Ishitani et al. (U.S. Pat. No. 4,939,364 July 1990) are concerned with preventing the bombarding ions from aggregating or segregating on a surface being processed by an ion beam. Rules for choosing the ion species are given and use of a mass spectrometer to monitor the extent of this problem is described.
Swann (U.S. Pat. No. 5,009,743 April 1991) supplements an ion beam with a directed jet of a reactive gas. The specimen being milled is also heated. This results in high milling rates for beams at near normal incidence.